R：在范围界定中，将参数传递给内部函数，以及

Question

The example below is another instance of my ongoing quest to be better at predicting what R will do. Despite having read the material on matching and scoping any number of times, I continue to have difficulty in passing variables, and it seems clear that there are either additional rules I am missing, or I am misunderstanding some of the rules I have, supposedly, learned.

Here ff() calls gg() four times. The output of these calls is below. Here instance 1 and 2 confirm my expectations. They find aa in the execution environment of ff , and cc in the enclosing environment of the function definition of gg . But instances 2 and 3 surprised me. For 3, I was not sure what would happen, as I am still a little foggy on whether a variable passed via ... retains its name. If not, I figured the garbage collector might get it. But I really expected 4 to pass a variable called cc with the value 13 into the function, and it does not.

My original version of gg() did not have the ls() the list2env() in it. When I added the ls() , it seemed to show that the ... passed a ... object, with the cc inaccessible inside it to the scope of the cc in the function body. Note that the ls() did not show the ... until I added the all.names=TRUE. I then added the list2env(), conditional on there being something in the ... . but as best I can tell, this does nothing at all.

So, three questions:

1. What is up with list2env? Why does it have no effect here?
2. How does one make named arguments (in the sense of an argument assigned a name by tag) in a ... generally available inside the function as if the name had been a formal?
3. Same question as (2), but with respect to a variable that has a name, or is a name, without that name being assigned via tag in the argument list.

aa <- 2
bb <- 3
cc <- 11
gg <- function(X, ...){
  if(length(alist(...)) > 1){list2env(alist(...), envir = parent.frame())} 
  env_vars <- ls(all.names=TRUE)
  out <-  cc * X
  print(env_vars)
  cat(X, ",  out = ", out, "\n", sep="")
}

ff <- function(){
  aa <- 5
  bb <- 7
  cc <- 13

  gg(aa)        \# 1
  gg(X=aa)      \# 2
  gg(aa, cc)    \# 3
  gg(aa, cc=cc) \# 4
}

ff()

[1] "..." "X"  
5,  out = 55
[1] "..." "X"  
5,  out = 55
[1] "..." "X"  
5,  out = 55
[1] "..." "X"  
5,  out = 55

A big thank you! to whoever fixed my formatting.

Answer 1

This does seem strange but with some trial and error we find that

L <- list(...)
list2env(L, environment())

will inject the ... variables into the current environment if the actual arguments are named. It seems that it is important to create an intermediate variable to hold the list, here L .

In general one could use with like this if the actual arguments are named:

g <- function(X, ...) with(list(...), {
   print(ls())
})
g(1, cc = 2)
## [1] "cc"